Theoretical structure of a magnetospheric plasma boundary: Application to the formation of discrete auroral arcs

Abstract

In the framework of a kinetic theory for tangential discontinuities we modeled the electrical structure of the sheath that separates magnetospheric particle populations of different densities and temperatures. The model can equally be applied to the plasma sheet boundary layer in the tail or to the boundary of some plasma sheet cloud immersed in the central plasma sheet. With plasma parameters typical of the Earth's outer magnetosphere and plasma sheet, we obtain results bearing many features pertinent to magnetospheric processes, specifically the origin of discrete auroral arcs. Creation of a space‐charge separation electrostatic potential in a direction normal to the magnetic field results from the contact of the two plasma populations. When the large‐scale solar wind potential difference is further imposed across the transition layer, the potential gradients are locally much enhanced, to give rise to large electric fields (several hundreds millivolts per meter) appearing over small distances perpendicular to the magnetic field—just the situation needed for the creation of an auroral arc. The transition itself is characterized by two scale lengths of the plasma and fields variables: the average electron Larmor radius ϱe (or some multiple of ϱe) for thin embedded electron‐dominated layers which generate the sharpest potential gradients, and the ion gyroradius ρp (or some multiple of ρp) for the broader ion‐dominated layers located at the outer edges of the transition. The larger‐scale sizes are appropriate to auroral arcs dimensions. The generated electric potential differences, consistent with the energy acquired by the precipitated electrons associated with discrete aurora, are identified with the source of the electromotive force (EMF) required for the auroral current circuit. Wave particle interactions are likely to scatter the electrons into the atmospheric loss cone, establishing the current system threading both the EMF and the ionosphere by means of field‐aligned currents. The half lifetime of the transition is at least of the order of 1000 s. This is also the time interval during which dissipative processes will not alter significantly the available potential gradients of the initially unloaded EMF.